Pumps of this general character have been proposed more than 100 years ago. For example in Ortmans German patent No. 1123 (1877), there is disclosed a rotary pump having a cylindrical body with plane end walls and suitable inlet and outlet pipes. At each of the two sides there is a gland through which extends a shaft on which there is a cast iron disc. The disc is formed of two spiral surfaces which are arranged symmetrically so that they lie one within the other and represent a wave-form surface of which the convex parts alternately remain in contact with the two end walls of the pump body. This spiral disc extends from its hub on the shaft to the periphery of the pump housing on which it tightly slides and by means of its position in the housing forms separate compartments. In order to avoid back flow of the fluid which moves with the rotating spiral disc, a a barrier slider is provided between the inlet and outlet openings. The slider comprises a plate provided with a slot perpendicular to the axis of the pump which accommodates the disc. Alternatively the slider comprises two hollow movable plates which are pressed by springs in the direction of the slot accommodating the disc. The spiral disc is gripped in the slot or between the two plates, which are pressed toward one another by the springs, and can move freely through the slot by reason of its uniform thickness and continuous surfaces and thereby automatically imparts a back and forth movement to the sealing slider.
Although pumps of this kind have important advantages over other types of pumps in that they are positive displacement rotary pumps requiring no valving, they have not come into use. This appears to be due to the fact that they present problems which have not been solved or perhaps even recognized. A primary problem is that of providing a fluid-tight engagement between nose portions of the sliders and the disc or rotor. This problem is more complex than may at first appear. Assuming that the rotor comprises a central hub portion from which an undulating vane portion extends radially, the undulations must have the same depth in an axial direction adjacent the hub as at the outer periphery of the rotor. However, the circumferential extent of the vane portion is much less adjacent the hub than at the periphery. This means that the slope of those portions of the undulations between the crests and bottoms of the undulations is steeper adjacent the hub than at the periphery. In fact, the steepness of the slope increases continously from the periphery of the vane to the hub.
If it is assumed that the noses of the sliders are knife edges which are radial and perpendicular to the axis of rotation of the rotor and that it is desired to maintain a constant spacing between the sliders, the opposite surfaces of the vane portions of the rotor will be perpendicular to the central plane of the sliders when, and only when, the sliders are at one of the crests of the undulations. At the crests, the thickness of the vane portion is equal to the spacing of the slider. In all other portions, the surfaces of the vane portion are not perpendicular to the central plane of the sliders and hence, if the spacing of the sliders is to be maintained constant, the thickness of the vane portion of the rotor must vary in accordance with the slope of the vane surfaces with respect to the central plane of the slider. This results in the thickness of the vane, at any given distance from the axis, being a minimum midway between opposite crests of the vane. Moreover, except at the crests of the undulations, the thickness must decrease from the periphery of the vane portion inwardly to the hub. The required configuration of the vane portion of the rotor is thus quite complex.
Moreover, from a practical point of view the noses of the sliders cannot be knife edges. If the noses of the sliders were sharp they would cause rapid wear of the rotor. Moreover, even if the noses of the sliders were initially knife edges they would quickly become blunted by wear. Moreover, it has been found that the pump will not "wear in" in the sense of the sliders and rotor wearing so as to conform to one another to provide a fluid tight engagement of the sliders with the rotor. Since the angle of engagement of the undulating vane with the sliders continually varies not only circumferentially but also radially, wear at one point results in non-engagement and hence leakage past the sliders at other points.
Since knife edges are undesirable from the point of view of wear and moreover, cannot be maintained the nose portions of the sliders must in practice be rounded. This complicates the problem of maintaing a fluid tight engagement of the sliders with the undulating vane portion of the rotor. Except at crests of the undulations, the line of engagement is no longer a line which is radial and perpendicular to the rotor and in fact is no longer a straight line but rather a three dimensional curve which varies from point to point. In order to provide a fluid tight engagement with the sliders, the contour of the surfaces of the undulating vane portion of the rotor must be modified to take into account the radius of the nose portion of the sliders.
A further problem in a rotary pump of this kind is to provide fluid tight contact between the rotor and the walls of the pump chamber. If fluid tight contact is not maintained, fluid leaks past the contact lines and the efficiency of the pump is decreased. Even though the pump is initially "tight", clearance develop sooner or later through wear of the parts. This is especially serious when pumping low viscosity fluids.